This invention relates to computer systems, and more particularly to a lock protocol for a computer system bus which uses a bridge between a processor bus and a standardized system bus.
Computer systems of the PC type usually employ a so-called expansion bus to handle various data transfers and transactions related to I/O and disk access. The expansion bus is separate from the system bus or from the bus to which the processor is connected, but is coupled to the system bus by a bridge circuit.
For some time, all PC's employed the ISA (Industry Standard Architecture) expansion bus, which was an 8-MHZ, 16-bit device (actually clocked at 8.33 MHZ). Using two cycles of the bus clock to complete a transfer, the theoretical maximum transfer rate was 8.33 MBytes/sec. Next, the EISA (Extension to ISA) bus was widely used, this being a 32-bit bus clocked at 8-MHZ, allowing burst transfers at one per clock cycle, so the theoretical maximum was increased to 33-MBytes/sec. As performance requirements increased, with faster processors and memory, and increased video bandwidth needs, a high performance bus standard was a necessity. Several standards were proposed, including a Micro Channel architecture which was a 10-MHZ, 32-bit bus, allowing 40-MByte/sec, as well as an enhanced Micro Channel using a 64-bit data width and 64-bit data streaming, theoretically permitting 80-to-160 MByte/sec transfer. The requirements imposed by use of video and graphics transfer on networks, however, necessitate even faster transfer rates. One approach was the VESA (Video Electronics Standards Association) bus which was a 33 MHZ, 32-bit local bus standard specifically for a 486 processor, providing a theoretical maximum transfer rate of 132-MByte/sec for burst, or 66-MByte/sec for non-burst; the 486 had limited burst transfer capability. The VESA bus was a short-term solution as higher-performance processors, e.g., the Intel P5 and P6 or Pentium and Pentium Pro processors, became the standard.
The PCI (Peripheral Component Interconnect) bus was proposed by Intel as a longer-term solution to the expansion bus standard, particularly to address the burst transfer issue. The original PCI bus standard has been upgraded several times, with the current standard being Revision 2.1, available from a trade association group referred to as PCI Special Interest Group, P.O. Box 14070, Portland, Oreg. 97214. The PCI Specification, Rev. 2.1, is incorporated herein by reference. Construction of computer systems using the PCI bus, and the PCI bus itself, are described in many publications, including "PCI System Architecture," 3rd Ed., by Shanley et al, published by Addison-Wesley Pub. Co., also incorporated herein by reference. The PCI bus provides for 32-bit or 64-bit transfers at 33- or 66-MHZ; it can be populated with adapters requiring fast access to each other and/or with system memory, and that can be accessed by the host processor at speeds approaching that of the processor's native bus speed. A 64-bit, 66-MHZ PCI bus has a theoretical maximum transfer rate of 528-MByte/sec. All read and write transfers over the bus can be burst transfers. The length of the burst can be negotiated between initiator and target devices, and can be any length.
System and component manufacturers have implemented PCI bus interfaces in various ways. For example, Intel Corporation manufactures and sells a PCI Bridge device under the part number 8245OGX, which is a single-chip host-to-PCI bridge, allowing CPU-to-PCI and PCI-to-CPU transactions, and permitting up to four P6 processors and two PCI bridges to be operated on a system bus. Another example is offered by VLSI Technology, Inc., is a PCI chipset under the part number VL 82C59 x SuperCore, providing logic for designing a Pentium based system that uses both PCI and ISA buses. The chipset includes a bridge between the host bus and the PCI bus, a bridge between the PCI bus and the ISA bus, an a PCI bus arbiter. Posted memory write buffers are provided in both bridges, and provision is made for Pentium's pipelined bus cycles and burst transactions.
The "Pentium Pro" processor, commercially available from Intel Corporation, uses a processor bus structure as defined in the specification for this device, particularly as set forth in the publication "Pentium Pro Family Developer's Manual" Vols. 1-3, Intel Corp., 1996, available from McGraw-Hill, and incorporated herein by reference; this manual is also available from Intel by accessing &lt;http://www.intel.com &gt;.
A CPU operates at a much faster clock rate and data access rate than most of the resources it accesses via a bus. In earlier processors, such as those commonly available when the ISA bus and EISA bus was designed, this delay in reading data from a resource on the bus was handled by wait states. When a processor requested data, and it was not immediately available due to a slow memory or disk access, then the processor merely marked time using wait states, doing no useful work, until the data finally became available. In order to make use of this delay time, a processor such as the P6 provides a pipelined bus that allows multiple transactions to be pending on the bus at one time, rather than requiring one transaction to be finished before starting another. Also, the P6 bus allows split transactions, i.e., a request for data may be separated from the delivery of the data by other transactions on the bus. The P6 processor uses a technique referred to as "deferred transaction" to accomplish the split on the bus. In a deferred transaction, a processor sends out a read request, for example, and the target sends back a "defer" response, meaning that the target will send the data onto the bus, on its own initiative, when the data becomes available. Another transaction available on the P6 bus is a "retry" response. If a target is not able to supply a requested item, the target may respond to the request from the processor using a retry signal, and in that case the processor will merely send the request again the next time it has access to the bus.
The PCI bus specification as set forth above does not provide for split transactions. There is no mechanism for issuing a "deferred transaction" signal, nor for generating the deferred data initiative. Accordingly, while a P6 processor can communicate with resources such as main memory that are on the processor bus itself using deferred transactions, it is not possible to employ this technique when communicating with disk drives, network resources, compatibility devices, etc., on an expansion bus.
A problem occurs for locked bus transactions because of the defer protocol on the P6 bus. In a deferred transaction, the P6 bus sends out a signal called "defer enable" or DEN#, indicating whether this transaction can be deferred. If it is required that this transaction complete in order, DEN# is not asserted, so the device must "retry" if it can't complete in order. If the target sees DEN# asserted, and it can't complete in order, its response is "defer" and the initiator (P6 processor) then initiates nothing further for this transaction; when the target has the data, it sends out a new address strobe on the bus, with a defer ID, and a defer reply. In a locked cycle, DEN# is not asserted; a locked cycle cannot be deferred. This creates a problem in guaranteeing forward progress for locked cycles going downstream; since locked cycles can't be deferred, the ordering rules make it difficult to get the lock cycle through on the PCI bus. For example, if a bridge implements locked reads on the P6 bus in immediate mode (i.e., without split transactions and using wait states), then it can not initiate a cycle on the PCI bus until the upstream write queue is empty. The posted writes can't be run on the processor bus because it is locked. One solution to this is to ensure that a locked cycle is not even tried unless there are no enqueued upstream posted writes; this is not a good solution because it depends on waiting for a fortuitous situation, which might not occur for a long delay, resulting in long periods of wait states on the CPU.